Adaptive arrays

ABSTRACT

Herein are disclosed multiple beam laser systems with adaptive phase control for establishing, at a target, an in-phase condition between the corresponding electromagnetic fields of all the beams. Phase modulation at different frequencies or differing waveforms is applied in the transmission paths of selected radiating elements of the array; and modulation components in the received energy are utilized to control phase shifters in the transmission paths so as to maintain the cophase condition at the target.

Waited Mates Patent UMeara 1 51 May 1, 973

541 ADAPTIVE ARRAYS 3,267,380 8/1966 Adams ..325/56 3,394,374 7/1968Weiss ..343/1OQ TD [75] Inventor Mean 3,611,381 10/1971 Preischat......343/100 TD [73] Assignee: Hughes Aircraft Company, Culver City, Calif.Primary ExaminerStephen C. Bentley [22] Filed: Feb 24, 1971 Attorney--W.H. MacAllister, Jr. and Lawrence V.

Lmk, Jr. [21] Appl. No.: 128,628

[57] ABSTRACT 150/199, 343/75, 343/100 TD, Herein are disclosed multiplebeam laser systems with 356/5 356/152 adaptive phase control forestablishing, at a target, an [51] lint. Cl. ..H04b 9/00 in phaseCondition between the corresponding e]ec [58] Field of Search ..250/l99;325/154,

325/159, 180, 368, 369, 56; 343/7 A, 7.5, 17.5,100 SA, 100 TD, 854, 208;356/4, 5,

tromagnetic fields of all the beams. Phase modulation at differentfrequencies or differing waveforms is applied in the transmission pathsof selected radiating 52 elements of the array; and modulationcomponents in the received ener are utilized to control hase shif- [56]References Cited g? 1 1 ters 1n the transm1ss1on paths so as to mamtamthe UNITED STATES PATENTS cophase condition at the target.

3,174,150 3/1965; Sferrazza .L .343 100 7 Claims, 6 Drawing FiguresFin'g'rii 12 Loser 4pc 506 22c 231C 2 c 14c r r 1 BF. Filter Phase A,Loser ntw Sen Det. l Ecps p 520 g "3 Phose Shifters 546 4236 59b 22b 216 2 5b I4b B\.P. Filter l huse Laser OM12 Sen.Det i ECPS- Amp "\54bPhase Shifter 48o 590 260 2 2 5u l I B.P. Filter Phase K Loser H. 0161SerLDet ECPS Amp Ut1|1mt|on Device 46 v .44 40 as 42 i P1566 Fnv LoserIF A Dlode Det. i l Local 05C} Patented May 1, 1973 3,731,103

'5 Sheets-Sheet 1 F i q. 1.

Phase Shifter l2 22 I8 24 3O 34 Phase .0 'r. Sens e 48 BPFiIter Env. 01w, Def.

Phase Shifters I 8| 24c Z from Loser l2 ECPS V Smgle Loser Transmit ECPSW power Telescope 4 Amplifier Control Electronics Units Thomas R.OMeclro, q INVENTOR.

M XMH ATTORNEY.

Patented May 1, 1973 5 Sheets-Sheet 2 ll q Btm 32a 2 ADAPTIVE ARRAYSBACKGROUND OF THE INVENTION This invention relates generally to adaptivearrays and particularly to such arrays wherein the relative phase ofenergy transmitted by a plurality of radiating elements is controlled toestablish an in-phase condition at the target.

The distribution of electromagnetic energy radiated by an apertureimmersed in the atmosphere differs .from the ideal diffraction limitedbehavior (assumes a vacuum) due to refractive index inhomogeneitiescaused by variations in atmospheric density. The angular width of themain lobe, its direction, and the intensity distribution within the lobeare all affected by these density variations. Among the principalsources of density variation are atmospheric turbulence and the heatingcaused by absorption of the radiated energy. In the case of atmosphericturbulence, the effect of the inhomogeneities depends on the strength ofthe turbulence and on the path length. As the strength of the turbulenceand/or the path length increases, the first noticeable effects arechanges in the direction of the beam (beam wander) and those associatedwith the random phase shift introduced across the beam (loss ofcoherence). The distribution of the radiated energy departs appreciablyfrom the ideal when the energy radiated from different parts of theaperture is no longer phase coherent at the receiving point.

For example, the performance of very narrow beam width (large aperture)coherent laser systems operating through the atmosphere is seriouslydegraded by atmospheric turbulence and also in some cases by nonlinearpropagation effects. Three turbulence related propagation effects are:the width of the main lobe of the radiation pattern is increased,reducing both resolution capability and power on target; the directionof the main lobe deviates from that predicted under free spaceconditions, and is not constant in time; and the shape of the radiationpattern may become highly irregular and time varying.

It is possible to reduce the deleterious effects of the atmosphere onthe radiation pattern of a large aperture by utilizing instead, an arrayof smallerapertures whose phase is adaptively controlled. If each of theindividual elements in the array are small enough that their radiationpattern is diffraction limited, near diffraction limited performancefrom the entire array may be obtained by adaptively changing therelative phase of the excitation sources driving various radiationelements, in such a manner that the atmospheric effects are compensated.The implementation of this concept requires the capability of sensingand changing the relative phase of the radiated energy at the target.

One adaptive compensation technique uses the phase difference betweensignals from multiple receiving channels, as determined by phasecomparison of the optical carrier frequencies (after heterodyneconversion), to control the required phase adjustments of thetransmitted beams. Although this technique may be a marked improvementover nonadaptive systems, it does not provide direct confirmation of acophase condition at the target. For example, if a phase measurementerror due to a path unbalanced exists in the system, then a cophasecondition will not be established at the target and hence atmosphericconditions are not fully compensated for by this technique.Additionally, this phase comparison between receiving channels approachrequires heterodyne detection that introduces mechanizationdifficulties, especially when targets of high doppler signatures areinvolved; and further problems are encountered with targets at suchranges that backscattered energy is comparable to energy reflected fromthe target.

SUMMARY OF THE INVENTION It is therefore an object of the subjectinvention to reduce the deleterious effects of the atmosphere on theradiation pattern of a large aperture by utilizing instead an array ofsmaller apertures which are adaptively controlled to maintain theplurality of transmitted beams in-phase at the target.

Another object of the subject invention is to provide an adaptive arraywhich directly senses the establishment of the cophase condition of allthe transmitted beams at the target.

A further object is to provide an adaptive array which does not requirea phase-matched, multi-channel, heterodyne receiver system.

Still another object is to provide an adaptive array which may be usedwith systems having a single receiving channel that need not have anycommon paths with any one of the transmitting channels.

Yet another object is to provide an adaptive array wherein thedefocusing effect of moving targets is substantially reduced.

In accordance with one preferred embodiment of the subject invention, adithering of variable phase shifters associated with the radiatingelements of an optical transmission array is employed such that each ofthe transmitted beams is phase modulated at a separate characteristicdither frequency. On reception of the energy from a target, envelopedetectors and filters centered at each of the dither frequencies providesignals for controlling a compensating phase shifter in eachtransmitting channel to provide an in-phase condition of all radiatedfields at the reflecting target. For each transmitting beam, themagnitude of the amplitude modulation components in the received energyat the associated dither frequency is indicative of the deviation of thephasing of its electromagnetic fields from a cophase condition at thetarget. The phase of the received amplitude modulated signals isindicative of the polarity of the phase error of the associatedtransmission channel.

The subject invention eliminates problems encountered by other systemsfor adaptively controlling arrays in that the cophase condition at thetarget is measured directly. Since the radiated field from each apertureelement of the array is characterized by its own signature its ditherfrequency separation of the received information into path errorcomponents associated with one and only one path is easier to mechani zeand it does not require heterodyne detection or complex computations.

BRIEF DESCRIPTION OF THE DRAWINGS The'novel features of this invention,as well as the invention itself, will be better understood from theaccompanying description taken in connection with the accompanyingdrawings in which like reference characters refer to like parts and inwhich:

FIG. I is a block diagram of a laser transmitting and receiving systemhaving an array adaptively controlled to establish a cophase conditionat the target, in accordance with the principles of the subjectinvention;

FIG. 2 is a diagram of the composite electromagnetic field at thetarget, for explaining the phase to amplitude conversion processutilized by the adaptive arrays of the subject invention;

FIG. 3 is a block diagram of a laser transmitting and receiving systemwherein each transmitting element of the array is adaptively controlledfor establishing an fin-phase condition of the energy at the target;

FIG. 4 is a block diagram of a portion of the laser system of FIG. 3with additional circuitry for maintaining an accurate phase referenceindependently of target range;

FIG. 5 is a block diagram of a phase compensated receiver that may beincorporated into the systems of FIGS. 1, 3 or 4; and

FIG. 6 is an optical array system which utilizes a single laser poweramplifier and telescope for transmitting a plurality of adaptivelycontrolled beams.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The subject invention may bebest understood by first considering the basic two element array of FIG.I. As there shown, the laser 12 excites transmitting aperture elements14 and 16 by means of transmission paths I8 and 20, respectively. Path18 includes a power. or beam splitter element 22 and an electronicallycontrollable phase shifter 24. Path includes beam splitter 22, a mirror26 and a beam splitter 28.

Transmitting apertures 14 and 16, which may include focusing optics suchas telescopes, radiate beams 30 and 32 respectively, so as to illuminatea target 34. A portion of the total energy reflected from target 34,shown as beam 36, is received by aperture 16 and is applied by means ofbeam splitters 28 and 38 to a mixer photodiode 40. This received energyis heterodyned in mixer 40 with an optical frequency signal supplied bya laser 42, and is amplified in an IF amplifier 44. The output signalfrom amplifier 44 is processed by an envelope detector 46 which producesan output signal that varies in amplitude in accordance with the envelope of the IF signal from amplifier 44.

The signal from detector 46 is processed by a passband filter 48centered at a frequency m and the output therefrom is applied as oneinput to a phase sensitive detector 50. The signal from phase detector50 has an amplitude A sin d), where A is a function of the magnitude ofthe signal applied from filter 50 and qb is the phase angle between thislast mentioned signal and a reference signal to applied from a referenceoscillator 52.

The signal from phase detector 50 is summed with the reference signal mby means of a transformer device 54; and this summed signal is appliedto an electronically controllable phase shifter 24. Phase shifter 24,which may be a movable mirror or an electro-optical device, for example,varies the effective transmission path length (or net phase shift) 18 inresponse to the signal applied thereto.

The optical signals which excite aperture elements 14 and 16 are ofacommon frequency but generally are not in phase. These two apertureelements radiate energy to a common target 34, and the fields from eachof these elements generally experience a further differential phaseshift at the target because of path length differences. The compositesignal at target 34 may be expressed as,

E,= 2A cos B, cos (m t +5 (2) where m id the frequency oflaser l2 and Bis the composite differential phase error. The constructive (ordestructive) interference of the two field components yields a spatialinterference pattern with a sinusoidal envelope 56 (FIG. 2). For thosecases where B varies with time, the spatial interference pattern wandersback and forth over the target region.

The modulation of electronically controllable phase shifter 24 producesa phase excursion,

pps( Bc Bm Sin "m where [3 is a corrective (unmodulated) phase shiftapplied from detector 50 to provide the desired phase adjustment. Thenet phase error 3,, attarget 34 is also dithered over this same range;that is,

fio fia fic flm m( where [3,, is the atmospheric (or other) phase errorto be corrected by B As a consequence, the interference pattern dithersback and forth over the target. Thus, the phase modulation produced byphase shifter 24, introduces an amplitude modulation at the ditherfrequency, in the composite signal on target and hence in the receivedreturn signal.

From another viewpoint, that of a fixed point (3 the envelope modulationprocess of Equation 3 is also illustrated in FIG. 2. For a phase errorB, to the right of the envelope peak (3,, B of phase error curve 56, themodulation envelope 58 is in phase with the dither source (oscillator 52in FIG. I). For a phase error B to I the left of the modulationenvelope, the modulation envelope 60 is out-of-phase. For B B thefundamental component of the modulation envelope vanishes. Thus, thearray of FIG. 1 has the required characteristics for a feedback controlsystem whereby the mean value of phase shifter 24, B is controlled suchthat B B is driven to zero, thereby establishing the cophase conditionat the target 34. In particular it is noted that the magnitude andplurality of the output signal of phase detector 50 is such as to forcethe mean phase value of path 18 to establish the cophase condition atthe target.

The extension of the target cophasing concept of the subject inventionto more than one phase controlled transmission path is illustrated inFIG. 3. In FIG. 3, each of the elements of the various transmissionpaths is given the same numeral designation as the corresponding elementof transmission path 18 of FIG. 1 with the letter a, b and c identifyingthe elements associated with transmission paths 62, 64 and 66respectively. Also in FIG. 3 laser amplifier 25 has been coupled in eachof the transmission paths between the phase shifters 24 and the apertureelements 14. Additionally, the received channel or signal path 68 isillustrated (for generality) in FIG. 3 with a separate receivingaperture 70 and beam directing mirror 72 associated therewith. In theoperation of the system shown in FIG. 3, the phase of eachelectronically controllable phase shifter (24a, 24b and 24c) is ditheredat its own characteristic frequency m The corresponding amplitudemodulation component in the reflected return signal (as explained aboverelative to FIG. 2) is separated, after detection, from the IF carrierby envelope detector 48; and from the other modulation frequencycomponents by means of bandpass filters centered on frequency mTherefore the phase correction offset, A introduced by each of theelectronically controllable phase shifters is a function of themodulation caused by the dither of that phase shifter only. The outputsignal of IF amplifier 46 is also coupled to a utilization device 47which may be a display or computer unit, for example, and which utilizesthe target information contained therein.

It is noted that I the single reference channel technique of the systemof FIG. 1 uses one of the transmission paths as an uncontrolledreference and the phase of the controlled channel is adjusted toestablish the cophase condition at the target. When this approach isextended to a large group of adaptively controlled radiating apertures,the extraction of the desired error information associated with each ofthe adaptive channels by simple filtering is complicated. This is due tointermodulation products resulting from the interaction of the phasemodulation applied to each of the plurality of transmitted beams. Oneapproach for avoiding this intermodulation problem is a sequentialswitching technique such that only the reference element and one otherelement then being established in phase synchronism, are radiating.After proper phasing is established for a particular controlled channelit is turned off and the same operation is repeated with the otherchannels. This switching sequence is performed at a repetition frequencyhigh enough that the sequence may be completed before there is anysubstantial change in the required phase corrections. Since eachtransmitting channel is cophased to the reference before a next elementis turned on, it would not be necessary to turn off the precedingelements. Therefore each element of the array may be phase adjusted by asequence which activates each of the controlled transmitting channelsone at a time.

As an alternative to the technique wherein the phase of each controlledtransmission path is determined with respect to a reference channel, thesystem of FIG. 3 measures and corrects the average phase error in eachtransmission path as compared to all other channels consideredsimultaneously. In the mechanization of FIG. 3, the synchronouslydetected component at a frequency m is a measure of the average of thesine of the phase errors of channel m compared to all other channels.Each error signal S (from the m" phase sensitive detector) can then beemployed to correct the phase error in the m" channel (feeding the m"radiating aperture). The distrubuted reference syst cm of FIG. 3 has oneimportant advantage in that the loss of any one signal component (A, 0)as a result of propagation or equipment failure does not destroy theoperation effectiveness of the remaining channels. The operation of thedistributed reference system may be compared to the common homodynedetection process. For this comparison the signal input is analogous tothe phase modulated signal from a particular transmitting channel, whilethe function of a reference oscillator is performed by the phasor sum ofthe ensemble of the remaining channels, as returned from the target.

The subject invention may be utilized to compensate for frequency errorsas well as phase error corrections inasmuch as frequency errors may beconsidered as phase errors which increase approximately as a linearfunction of time. Such frequency errors could result from oscillatordrifts or differential doppler shifts, for example. As used herein theterm differential doppler shift means the difference in the dopplerfrequencies between transmitted beams.

Although in the transmission systems shown in FIGS. 1 and 3, the energyradiating from each of the transmitting apertures originates from acommon laser source, in an alternate embodiment a separate laseroscillator may be used in each of the transmitting channels. Theoperation of the system would remain unchanged except that thecorrection voltages, S would be applied to a frequency controllingelement,

such as a mirror internal to the laser cavity, rather than a phasecontrolling element. This technique circumvents the accumulation ofexcess phase shift by changing the transmitted frequency of each arrayelement to compensate for frequency errors. An increase in cost may beassociated with this mechanization due to the need for a plurality oflaser oscillators, although the beam splitting elements would beeliminated. Also, each laser oscillator iivould have to be supplied withits own"self-stabilizing loop to control its frequency during search andacquisition of targets. The frequency stability of the plurality oflasers must be such that they stay within the capture range of thecontrol loops.

If the dither phase modulation frequencies (0), are selected too lowthen there is the possibility of interference from target or atmosphericscintillation modulations. On the other hand, if the labeling (tagging)modulations employed aretoo high a problem with the phasesynchronization between the dither signal reference and the returnsignal may result. Hereinabove for the purposes of explaining thefundamental principles of the invention it has been assumed that eitherthe beam labeling frequencies (m were sufiiciently low, or that thepropagation paths are sufficiently short that for all practical purposesthe phase detection operation could be considered to be synchronous.That is, it was assumed that there was no time slip or phase errorbetween the received input signal to the phase sensitive detector, suchas detector 50a of FIG. 3, for example, and the reference signal appliedfrom the reference oscillator, such as 52a.

In the more general 'case, where there exists a substantial round tripdelay 1' in the received signal, the control voltage S from the m'"phase detector is a function of cos (m -r). If w,,.r becomes as large as1r/4 the signal S vanishes independently of phasing errors at thetarget. If m 'r equals 'rrl, where I is an odd integer, the sign of theerror signal S reverses and the system will lock on the minima ratherthan the maxima of the composite beam (56 of FIG. 2). Therefore, in anuncompensated system the value of m must be restricted. Such alimitation on the value of the term w r determines the highestpermissible value of w, for a given maximum range. Since high modulationfrequencies are sometimes desirable to minimize noise interferenceproblems or to accommodate large frequency errors it may be desirable toincorporate techniques to compensate for the round trip delay time (1').

One method of compensating for the time displacement between thereference signal to the synchronous detector (phase sensitive detector50, for example) and the return modulated envelope (waveform 58 of FIG.2) would be by knowing or measuring the range to the target andcomputing the associated phase delay value. This phase shift could thenbe compensated by introducing a phase delay correction in the path ofthe reference signal to the synchronous detector.

A more generally applicable method of compensating for the loss ofsynchronism between the reference to the control loop phase detector andthe modulated return signal is to transmit the reference ditherfrequency (w,,,) in the form of modulation on some car rier'and toprocess the associated return signal so that this transmitted referencemay be used as the reference for the synchronous detector. Since boththe input signal to the phase sensitive detector and the referencethereto would then experience the same path delays, a purely synchronousdetection operation can be obtained independently of the value of theterm m 'r. For the system such as shown in FIG.'3, the most convenientchoice for carrier and modulation types would be those which are alreadypresent, that is, phase or frequency modulation of the optical carriers.FIG. 4 shows one transmit channel and the received channel of FIG. 3modified to compensate for the above described synchronization problem.The transmitted channel in FIG. 4 is designated generally by thereference numeral 62 and elements thereof corresponding to like orsimilar element of channel 62 of FIG. 3 are, in FIG. 4, identified bylike reference numerals. Only one modified transmission channel isill'us trated in FIG. 4 for explaining the transmission path delaycompensation technique. However, in practice the compensationmodifications would be incorporated into each of the transmissionchannels such as channel 64 and 66 of FIG. 3, in a manner identical tothat to be described for channel 62'.

Referring now primarily to FIG. 4, optical energy from laser 12 (FIG. 3)is modulated by electronically controllable phase shifter 24a andtransmitted by aperture 14a. The energy transmitted from aperture 14aalong with the energy from the other transmitting apertures (not shownin FIG. 4) is reflected from target 34. A portion of this reflectedenergy is intercepted by antenna 70 and processed by means of mirror 72,beam splitter 38, photodiode mixer 40 and laser local oscillator 42 inthe same manner as described relative to FIG. 3. The output of mixer 40after being processed by IF amplifier 46 is applied in parallel to anenvelope detector 48 and a limiter 78. The limiter 7B removes theenvelope modulation effects and the output signal therefrom isdemodulated by means of a conventional frequency discriminator 80. Theoutput from discriminator is applied in parallel to a bandpass filter 82at a frequency 01 associated with the transmission channel 62, as wellas to similar filters which are at the frequencies associated with theremaining transmitting channels (not shown). For example, if a channel64' and 66' were shown the output of frequency discriminator 80 wouldalso be processed by bandpass filters at frequencies (0 and 00respectively.

The output signal of frequency discriminator 80 contains all of thelabeling frequencies (m with a time delay corresponding to thetransmission-reception path of the received signal. The output signalsfrom the bandpass filters, such as filter 82 associated withtransmission channel 62, could be used to reference the phase sensitivedetectors, such as 500, of the associated channel. In the embodiment ofFIG. 4, however, the output of the bandpass filter is utilized tocontrol a phase locked loop 84 which includes a phase detector 86, aloop filter 88 and a voltage controlled oscillator 90. The phasedetector 86 compares the phase between the oscillator 90 and the outputsignal from the filter 82; and the output signal of phase detector 86,after being processed by loop filter 83, is used to adjust the voltagecontrolled oscillator 90 such that the frequency and phase of oscillator90 tracks the signal from filter 82. The output signal from theoscillator 90 is also applied to the phase sensitive detector 50awherein it is utilized as a reference for the received, modulated signalassociated with the frequency to, applied to detector 50a through thefilter 48a.

The output signal of phase sensitive detector 50a is summed with thesignal of a reference oscillator 52a by means of a transformer device540. This summed signal is applied to control electronicallycontrollable phase shifters 24a to apply phase modulation to thetransmitted signal at the labeling frequency (0 and to adjust the meanvalue of the phase of the channel such that it is in phase with theother transmission channels (not shown in FIG. 4) at the target 34.

As is now evident, the systems in accordance with the subject inventionfunction equally well with a separate or even remotely located receivingaperture. This characteristic is desirable in certain applications for anumber of reasons, not the least of which are the elimination ofback-scattering and crosscoupling problems inherent in systems thattransmit and receive from the same apertures. However, a certain amountof economy is sometimes realized by the transmitting and receivingsystems sharing apertures and the subject system is of course adaptableto a common transmit and receive mechanization, if so desired.

In some mechanizations it may be desirable to extend the size of thereceiving aperture beyond its coherent length in order to improvesignal-to-noise ratios for distance targets. In such cases adaptivecontrol of a receiving array may be employed.

One approach to adaptive control of the receiving system, if commontransmission and receiving apertures are employed, would be to use thephase error information, S extracted from the transmitter controlportion of a multi-dither system to correct (in an openloop manner) forthe phase errors in the received paths. However, this approach suffersfrom the problem that the transmitter system corrects for all phaseerrors in the transmission path including any laser power amplifiersthat may be utilized, as well as those in the post-aperture, radiationpaths. Also, since such an approach requires media linearity it isrestricted to low levels of transmitterpower.

Another approach is to employ an adaptively controlled receiving systemwith a separate phase modulation labeling frequencies associate witheach receiving channel in a manner analogous to the above describedtransmitter system. However, this method requires the doubling of thenumber of dither frequencies on transmission and thereby increases theproblem of avoiding possible intermodulations.

The method of adaptively controlling a plurality of receiving channelswhich may be best adapted to a large variety of applications involvesmeasuring the phase differences between IF receiver channels and usingthis difference to control phase shifters in the received channels suchthat the phase differences between channels are driven to zero. One suchmechanization is shown in FIG. wherein a plurality of receivingapertures corresponding to the aperture 70 of FIGS. 3 and 4 aredesignated by reference numerals 70a, 70b, 70c and 70d. In like mannerthe IF amplifiers for each channel which would correspond to amplifier44 in the previously described embodiments are given the same referencenumeral with a postscript corresponding to the particular channel. Inthe embodimerit of FIG. 5 varactor diode phase shifters are insertedfollowing the IF amplifiers in each controlled channel prior to asummation circuit 94. Summation circuit 94 forms the sum of the receivedsignals which have previously been translated to the intermediatefrequency zone and phase adjusted to be in phase. The varactor phaseshifters associated with each of the controlled receiving channels aredesignated by the reference numeral 92 with the postscript of thecorresponding channel. Phase detector 96 compares the phase differencesbetween channel d and channel a and drives the phase shifter 92a to nullthis difference. Similarly, phase detector 98 compares the phasedifference between channels b and d and controls phase shifter 92b inresponse thereto; and phase detector 100 compares channels c and d andcontrols phase shifter 92c. Hence the circuit of FIG. 5 adjusts thephase of the received signals to a cophase condition prior to theirsummation in circuit 94. The output signal from summation circuit 94 maybe applied to a utilization device 47, such as a display or computationsystem, as well as to envelope detector 46 (FIG. 3) which feeds thetransmitter control loops.

In the disclosed embodiments heterodyne detection (mixer 40) wasutilized because in general the detection is better than if videodetection without prior IF amplification were used. However, the subjectinvention is equally well adapted to noncoherent detection and in someapplications such as those involving high doppler frequency shifts, forexample, noncoherent detection may be preferred. For example, heterodynedetection systems require either that the IF circuitry has sufficientbandwidth to accommodate target doppler shifts or else the dopplerfrequency must be tracked out of the systemprior to the IF circuitrysuch as by a tuneable local oscillator which is adjusted closed loop tocenter the received IF spectrum at a selected frequency.

In the enclosed embodiments the labeling modulation (m has been appliedby the same unit, e.g. phase shifter 24, that applies the phasecorrections, [3,, however it will be recognized that these two functionsneed not be mechanized by a single unit. Hence, the phase modulation maybe applied by one electronically controllable device and the phasecorrection to maximize the power on target by another.

For applications involving targets having very slow angular rates, theelectrically controllable phase shifters such as element 24, maycomprise conventional piezoelectrically driven reflective mirror typephase shifting devices. One such phase shifter, constructed of a discshaped piezoelectrically driven bimorph material having a small thinmirror of approximately 5 millimeter diameter, for example, bonded atits center was found to have an extended frequency range beforemechanical resonances were bothersome. In applications requiring thephase shifters to have a frequency response above those convenientlyobtainable with mechanically or piezoelectrically driven devices,electro-optical phase shifters may be utilized.

In dynamic system applications, a target may be designated to theoptical array system at some precisely defined angle and angular rate.Each of the radiating optical aperture elements in the array may have amechanical steering mechanism (not shown) capable of its own autonomoussearch, acquisition and track functions. If boresight accuracy issufficiently high the preliminary search may be performed by only one ortwo of the radiating channels. After all the elements have separatelyacquired the target, and are tracking it, the adaptive control loops maybe activated and the adaptive array pattern forming and trackingcommenced.

In an alternate embodiment shown in FIG. 6, the energy from a laser 12is applied to electronically controllable phase shifters 24a, 24b, and24c by means of lenses 71 and 73. The output beams from the phaseshifter units are applied by lenses 75 and 77 to a laser power amplifier79. The amplified, beams from amplifi: er 79 are transmitted by a singletelescope 81. Each of the beams are phase modulated at separatefrequencies W,,, and are adaptively controlled, in a manner similar tothat explained above relative to FIG. 3, in response to signals appliedto each of the phase shifters from a control electronic unit 83. Unit 83includes a receiving channel such as 68 of FIG. 3, as well as theassociated processing devices such as 48, 50, 52 and 54 of FIG. 3. Alsofor electronic scanning applications the phase scanning control signalmay be provided by unit 83 and superimposed on, or time shared with, thesignals applied to the phase shifter unit 24. Although in the interestof clarity of the drawing only a single lead is shown from unit 83, itis understood that in practice a pair of leads may be coupled betweeneach of the phase shifters and unit 83. The embodiment of FIG. 6 reducesthe number of high power laser sources required and by use of a singletransmitting aperture simplifies boresighting and angle trackingproblems.

Thus there has been described new and improved adaptive arrays whereinmultiple, time varying perturbations, for example, phase dithers, areintroduced on transmission. The effects of these perturbations aresensed on reception and employed to control feedback loops which adjustthe phasing of the plurality of transmitting channels such that theenergy at the target is in phase. Some of the advantages realized bysystems incorporating the principles of the subject invention are: phasecoherency at the target is directly measured rather than being inferredby multiple receiving channel phase measurements; either noncoherent orcoherent detection systems may be employed; receiving optics may belocated anywhere, thereby avoiding backscattering and cross-couplingproblems; the ensemble reference mechanization provides a fail safesystem in the sense that the array functions correctly with theremaining elements in the event of the failure of one or more radiatingchannels; and the defocusing effects induced by moving targets aresubstantially reduced.

What is claimed is:

1. The method of transmitting a plurality of beams of optical energysuch that the plurality of beams are substantially in phase at a target,said method comprising the steps of:

providing a plurality of beams of coherent optical energy;

modulating each said beam at a different frequency by varying thetransmission path delay of each of said beams at the modulatingfrequency of that beam;

transmitting said plurality of modulated beams at a target;

receiving a portion of the energy reflected from the target; and

controlling the mean phase of each of said beams to cause the amplitudemodulation components in the received energy, which are at approximatelythe same frequency as the modulating frequency of that beam, to benulled.

2. A system for transmitting a plurality of beams of optical energy andfor controlling their relative phase so that the beams are substantiallyin phase at a remotely located target, said system comprising:

a laser;

an array of electronically controllable optical phase shifters;

a laser power amplifier;

lens means for applying the output beam from said laser through saidarray of optical phase shifters to said laser power amplifier;

identifying modulation means for applying modulation drive signals at adifferent frequency to each of said electronically controllable opticalphase shifters, whereby each of the output beams from said phaseshifters are phase modulated at different modulation frequencies;

telescope means for transmitting the output beams from said laser poweramplifier towards'the target; receiving means for receiving a portion ofthe transmitted energy reflected from the target; and

control means responsive to modulation signal components in the receivedenergy for controlling the mean phase of each of said electronicallycontrollable optical phase shifters so as to null the modulation signalcomponents in the received energy which are at the frequency of themodulation drive signal applied to the respective optical phase shifter;whereby the phase of said plurality of beams is adaptively controlled sothat said beams are substantially in phase at the target.

3. The system of claim 2 wherein said identifying modulation meansincluding a different reference oscillator for supplying the modulationdrive signal to each of said electronically controllable optical phaseshifters; and said control means including a plurality of controlcircuits, with each control circuit coupled for controlling the phase ofa different one of said beams and comprising a filter having a passbandcentered at the frequency of the modulation drive signal of theassociated beam, and means for applying amplitude modulation signalcomponents of the received energy to said filter, a phase sensitivedetector having a signal input coupled to the output of the filter ofthat control circuit, a reference input coupled to the output of thereference oscillator which supplies the modulation drive signal for theassociated beam, and an output coupled to the electronicallycontrollable optical phase shifter disposed in the transmission path ofthe associated beam.

4. The system of claim 3 wherein each of said electronicallycontrollable phase shifters is a piezoelectrically driven mirrorelectrically coupled to the associated reference oscillator for applyingphase modulation, and is electrically coupled to the output of theassociated phase detector for adjusting the mean phase value of saidbeam.

5. The device of claim 3 wherein each said phase shifters is anelectro-optical device electrically coupled to said reference oscillatorof the associated beam for providing phase modulation of said beam, andis electrically coupled to the output of said phase detector forcorrecting the mean phase value of said associated beam.

6. The system of claim 2 wherein said receiving means comprises aplurality of receiving circuits; means for sensing a phase differencebetween signals in said plurality of receiving circuits; and means foradjusting the phase delay within said plurality of circuits to null thephase difference.

7. The system of claim 3 wherein said receiving means comprises aplurality of receiving circuits; means for sensing a phase differencebetween signals in said plurality of receiving circuits; and means foradjusting the phase delay within said plurality of circuits to null thephase difference.

1. The method of transmitting a plurality of beams of optical energysuch that the plurality of beams are substantially in phase at a target,said method comprising the steps of: providing a plurality of beams ofcoherent optical energy; modulating each said beam at a differentfrequency by varying the transmission path delay of each of said beamsat the modulating frequency of that beam; transmitting said plurality ofmodulated beams at a target; receiving a portion of the energy reflectedfrom the target; and controlling the mean phase of each of said beams tocause the amplitude modulation components in the received energy, whichare at approximately the same frequency as the modulating frequency ofthat beam, to be nulled.
 2. A system for transmitting a plurality ofbeams of optical energy and for controlling their relative phase so thatthe beams are substantially in phase at a remotely located target, saidsystem comprising: a laser; an array of electronically controllableoptical phase shifters; a laser power amplifier; lens means for applyingthe output beam from said laser through said array of optical phaseshifters to said laser power amplifier; identifying modulation means forapplying modulation drive signals at a different frequency to each ofsaid electronically controllable optical phase shifters, whereby each ofthe output beams from said phase shifters are phase modulated atdifferent modulation frequencies; telescope means for transmitting theoutput beams from said laser power amplifier towards the target;receiving means for receiving a portion of the transmitted energyreflected from the target; and control means responsive to modulationsignal components in the received energy for controlling the mean phaseof each of said electronically controllable optical phase shifters so asto null the modulation signal components in the received energy whichare at the frequency of the modulation drive signal applied to therespective optical phase shifter; whereby the phase of said plurality ofbeams is adaptively controlled so that said beams are substantially inphase at the target.
 3. The system of claim 2 wherein said identifyingmodulation means including a different reference oscillator forsupplying the modulation drive signal to each of said electronicallycontrollable optical phase shifters; and said control means including aplurality of control circuits, with each control circuit coupled forcontrolling the phase of a different one of said beams and comprising afilter having a passband centered at the frequency of the modulationdrive signal of the associated beam, and means for applying amplitudemodulation signal components of the received energy to said filter, aphase sensitive detector having a signal input coupled to the output ofthe filter of that control circuit, a reference input coupled to theoutput of the reference oscillator which supplies the modulation drivesignal for the associated beam, and an output coupled to theelectronically controllable optical phase shifter disposed in thetransmission path of the associated beam.
 4. The system of claim 3wherein each of said electronically controllable phase shifters is apiezoelectrically driven mirror electrically coupled to the associatedreference oscillator for applying phase modulation, and is electricallycoupled to the output of the associated phase detector for adjusting themean phase value of said beam.
 5. The device of claim 3 wherein eachsaid phase shifters is an electro-optical device electrically coupled tosaid reference oscillator of the associated beam for providing phasemodulation of said beam, and is Electrically coupled to the output ofsaid phase detector for correcting the mean phase value of saidassociated beam.
 6. The system of claim 2 wherein said receiving meanscomprises a plurality of receiving circuits; means for sensing a phasedifference between signals in said plurality of receiving circuits; andmeans for adjusting the phase delay within said plurality of circuits tonull the phase difference.
 7. The system of claim 3 wherein saidreceiving means comprises a plurality of receiving circuits; means forsensing a phase difference between signals in said plurality ofreceiving circuits; and means for adjusting the phase delay within saidplurality of circuits to null the phase difference.